Phase stops are used in interferometers to modify the phase of a portion of a light beam passing therethrough. These phase stops are often used in common-path interferometers such as the Zernike phase contrast microscope. Among the advantages of common-path interferometers over separate-path interferometers is that the former tend to be more rugged and less susceptible to environmental perturbations due to mechanical, thermal and other similar forces as well as being less susceptible to aberrations caused by the additional optical components required by the separate-path interferometers. The use of a variable diameter dynamic amplitude pin hole design for use in separate-path interferometers has been suggested for the purpose of increasing fringe visibility. Although this arrangement is useful, it is relatively inefficient since the light of interest is diminished due to photon loss in the beam splitters in the system and the amplitude-stop nature of the pin hole design.
The known common-path interferometers also have their drawbacks. Since they generally use fixed-diameter phase stops, the known systems can only be optimized for a limited range of low to high spatial frequency content in the wavefront. Accordingly, fringe visibility will not be very good if it is desired to use the interferometer to analyze a different class of phase objects than the phase object for which the fixed phase stop was selected, or phase objects that vary in time.